Active filters are employed for a variety of fluid filtering applications. The term "filter," as used herein, is intended to encompass any device that blocks, traps and/or modifies particles or molecules passing through the device. The use of the term "fluid" in the present invention is intended to encompass any form of readily flowing material, including liquids and gases. The term "active" shall mean that the filter is capable of action upon one or more components, or "target species," of a fluid stream, whether by catalysis, reaction, or some combination thereof, so that a modified specie(s) is formed. Typically, these filters combine an active material, such as a catalytic material (e.g., TiO.sub.2, V.sub.2 O.sub.5, WO.sub.3, Al.sub.2 O.sub.3, MnO.sub.2, zeolites, and transition metal compounds and their oxides) and/or reactive material (e.g., sodium hydroxide, etc.) within a matrix. As the fluid passes over or through the matrix, target species within the fluid will react with active sites of the active material to convert the target species to a more desirable by- or end-product, and therefore remove the target species from the fluid stream. The term "active site," as used herein, describes a location on the active material where interaction with a target species occurs. Examples of such include:
Target species Active Material Resulting Product(s) NO.sub.x, NH.sub.3 TiO.sub.2, V.sub.2 O.sub.3, WO.sub.3 N.sub.2 + H.sub.2 O CO Al.sub.2 O.sub.3, Pt CO.sub.2 Dioxin/Furan TiO.sub.2, V.sub.2 O.sub.3, WO.sub.3 CO.sub.2, HCl, H.sub.2 O O.sub.3 MnO.sub.2 O.sub.2 CO.sub.2 Na(OH) Na.sub.2 CO.sub.3
Examples of previous attempts to produce a catalytic filter device include those set forth in U.S. Pat. Nos. 4,220,633 and 4,309,386, to Pirsh, where filter bags are coated with a suitable catalyst to facilitate the catalytic reduction process of NO.sub.x. In U.S. Pat. No. 5,051,391, to Tomisawa et al., a catalyst filter is disclosed which is characterized in that catalyst particles which are made of metal oxides with a diameter of between 0.01 to 1 um are carried by a filter and/or a catalyst fiber. In U.S. Pat. No. 4,732,879, to Kalinowski et al., a method is described in which porous, preferably catalytically active, metal oxide coatings are applied to relatively non-porous substrates in a fibrous form. In patent DE 3,633,214 A1, to Ranly, catalyst powder is incorporated into multilayered filter bags by inserting the catalyst into the layers of the filter material. Further examples to produce catalytic filter devices include those set forth in JP 8-196830, to Fujita et al., in which a micropowder of an adsorbent, reactant, or the like is supported in a filter layer interior. In JP 9-155123, to Sasaki et al., a denitrification layer is formed on a filter cloth. In JP 9-220466, to Kaihara et al., a catalyst filter is made by impregnating a cloth of glass fibers with titanium oxide sol which is then heat treated and further impregnated with ammonium metavanadate. In JP 4-219124, to Sakanaya et al., a compact, thick, and highly breathable filter cloth is filled with catalyst for the bag filter material in order to prevent catalyst separation. In U.S. Pat. No. 5,620,669, to Plinke et al., the filter comprises composite fibers of expanded polytetrafluoroethylene (ePTFE) having a node and fibril structure, wherein catalyst particles are tethered within the structure.
In most cases of the above-mentioned patents (e.g., in JP 9-155123, JP 9-220466, JP 4-219124 and U.S. Pat. Nos. 4,220,633 and 4,309,386), the filters are capable of collecting substantial amounts of dust, such as that generated in a combustion process. Conventionally, after short collection times (on the order of minutes to hours), a layer of collected dust on the dirty side of the filter material increases the pressure drop across the filter, and the filter has to be cleaned (in many cases in situ). During this cleaning cycle (e.g., a high energy air impulse system, a shaker system, a reverse air system, etc.), the outer dust layer falls off and a new filtration cycle can begin. During the operational process, two main problems can occur, namely chemical deterioration and mechanical deterioration.
With chemical deterioration, the chemical function of the filter can be rendered useless due to contamination, which is a serious problem with virtually every previous active filter device, and especially for catalytic filter devices. Although, by definition, catalysts are not consumed during the catalytic reaction, catalytic filters may have limited operating lives due to particle, liquid, and gaseous contamination from a fluid stream (i.e., fine dust particles, metals, silica, salts, metal oxides, hydrocarbons, water, acid gases, phosphorous, alkaline metals, arsenic, alkali oxides, etc.). Deactivation occurs because the active sites on the active particles are physically masked or chemically altered. Unless these contaminants can be shed from the filter, the filter will rapidly diminish in efficiency until it must be replaced.
As has been noted, a variety of cleaning apparatus exist to remove dust from filters (e.g., shaker filter bags, back-pulse filter bags and cartridges, reverse air filter bags, etc.), but these devices are not particularly effective at removing dust embedded inside the filter material.
Another need in catalytic filtration is to protect the active sites on the active particles from condensing fluids. These fluids, which often are heavily contaminated with heavy metals, can condense on the surface of filter materials during the normal operation, particularly such as in the case of combustion plants. If these liquids come into direct contact with the active material, they can severely contaminate the active sites and render those sites less active.
Another form of chemical deterioration is due to the loss of inserted active particles during operation. The active particles in many instances are not attached strongly enough to the host fibers to withstand the rigors of normal operation. The active particles fall out of the filter, thereby not only diminishing filter effectiveness, but also contaminating the clean fluid stream.
With respect to mechanical deterioration, the mechanical function of the filter can deteriorate by abrasion of the filter fibers during operation or by penetration and collection of dust contaminates in the filter. Another mechanical failure is due to dust particle break-through which occurs with certain of the filters in the art cited above. This phenomenon can be observed especially during the cleaning cycles in which particles slowly migrate through and out of the filter medium due to rigorous filter cleaning.
Further, the mechanical function can be hindered when high active particle loadings of the substrate increase the pressure drop of the filter excessively and/or the filter material becomes too stiff to be handled.
Most prior art publications describe products which in some way address the chemical deterioration. In some patents (e.g. JP 9-155123), the catalyst layer is located on the downstream side of the filter, thus avoiding exposure of the catalyst to particulate contamination. In other cases, such as JP 9-220466, it is recognized that the catalyst will decrease in activity due to particulate and chemical contamination and other such factors when the catalyst is used for an extended period of time. One anticipated disadvantage of this structure is that the effect of generation of active sites is negated when physical masking of the pores by gas stream contaminates renders the active sites in the pores useless.
In two cases, namely JP 8-196830 and U.S. Pat. No. 5,620,669, the contamination of the catalyst by particulate and gaseous contaminants is not believed to be a problem. Both of these filter materials have a protective layer to avoid such contamination. However, the material described in JP 8-196830 has several disadvantages. First, it is rather thick because of the thickness of the protective layer. One anticipated disadvantage of such a filter is that due to the thickness, it will be stiff and, thus, expected to be difficult to clean by traditional cleaning methods. In addition, the active particles of the described product are not mechanically attached to the fibers of the filter material. The zeta potential difference of the catalyst particles to the polymeric fibers is relied upon for the attachment mechanism. Attrition of the catalyst particles is described in the document.
Japanese Patent Application No. 10-230119, assigned to ABB Co., Ltd. is directed to a filter material formed by immersing fibers to be formed into a filter cloth in a liquid catalyst, drying the catalyst, molding the fibers into a filter cloth and applying a ethylene tetrafluoride resin continuous porous thin film to the filter cloth. The ethylene tetrafluoride resin film is relied upon to keep the catalyst from flaking off of the filter cloth. However, no mention is made as to how catalyst particles in regions which are not adjacent the thin film are kept from flaking off.
The only commercial filter material available to date which offers combined particulate and target species removal is taught in U.S. Pat. No. 5,620,669, described earlier. This filter combines the concept of catalyst particle protection by a microporous membrane and the incorporation of catalyst particles attached directly to nodes and fibrils for strong adhesion and low pressure drop. Unfortunately, this filter material can be rather complicated and expensive to manufacture.
Accordingly, it is a primary purpose of the present invention to provide an improved active filter material that is effective at catalyzing and/or chemically reacting target species of a fluid stream.
Another purpose is to provide a filter active material wherein the active particles are tethered to a porous substrate by at least one polymer adhesive selected from the group consisting of polytetrafluoroethylene (PTFE); fluoroethylene propylene (FEP); high molecular weight polyethylene ((HMW-PE)i.e., where the molecular weight is 1 million or greater); high molecular weight polypropylene ((HMW-PP) i.e., where the molecular weight is 1 million or greater); perfluoroacrylate (PFA); polyvinylidene fluoride (PVDF); a terpolymer of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride (THV); and polychloro trifluoro ethylene (CFE).
It is yet another purpose of the present invention to provide an improved active filter material that can be effectively cleaned, with minimum loss of activity of the active material, so that the filter material has an extended effective operating life.
It is yet another purpose of the present invention to provide a method of manufacturing an improved active filter material which has minimal effect on the activity of the active material contained within the filter material.
It is yet another purpose of the present invention to provide an improved active filter material that is easily and inexpensively manufactured.
These and other purposes of the present invention will become evident based upon a review of the following specification.